Statistics show that 1 out of every 7 females in the US will develop breast cancer in their lifetime, with 75-95% of patients with advanced breast cancer having chronic, excruciating pain associated with metastasis of the cancer to bone. Pain from metastatic breast cancer appears to be simultaneously driven by inflammatory, neuropathic and tumorigenic mechanisms. Such pain is extremely difficult to treat causing quality of life in these patients to be severely compromised. Being able to more fully control bone cancer pain, without the side effects of currently available analgesics, would significantly improve the functional status, quality of life while reducing health care costs in breast cancer patients with skeletal metastases. A major problem in designing new therapies to treat breast cancer-induced bone pain has been the lack of a model available to define the mechanisms that generate and maintain breast cancer induced bone pain. The major thrust of this proposal is to use a mouse model we have recently developed that closely mirrors the bone remodeling and chronic pain often observed in patients with breast cancer- induced bone pain. Mouse 66.1 breast cancer cells, stably transfected with green fluorescent protein, are injected and confined to the intramedullary space of the femur of the non-immunocompromised mice. Over a twenty-one day period, these tumor cells proliferate in the marrow space and induce bone remodeling, bone cancer related pain behaviors and ultimately fracture of the bone. Recent data has suggested that cannabinoid CB2 agonists can provide significant analgesia in a variety of preclinical pain models with a minimal side effect profile. Intriguingly, data from osteoporosis studies have suggested that activation of CB2 receptors is pro-osteogenic (bone-building) and preliminary data from our lab suggests CB2 agonists have direct anti-tumor effects in both mouse and human breast cancer cells. In the present proposal, we will explore the hypothesis that CB2 agonists can have multiple beneficial actions to reduce breast cancer-induced bone pain, reduce tumor induced bone destruction and fracture and reduce the growth of breast cancer cells both in vitro and in vivo. The specific hypotheses to be tested are: CB2 receptor activation will result in antihypersensitivity in a murine model of breast-induced bone cancer pain while lacking the unwanted side effects seen with current analgesic therapies. Experimental studies proposed in this application will: 1) aid in our understanding of the pain relieving effects of the CB2 receptors in breast-induced bone cancer, 2) identify whether CB2 agonists will enhance bone remodeling and reduce bone fracture in a murine model of bone cancer using breast cancer cells, 3) identify whether CB2 agonists inhibit the proliferation of breast cancer cells both in vitro and in vivo, 4) identify whether CB2 receptor activation results in a decrease in pronociceptive factors including IL-12, IL-6, and TNF1, 5) identify whether chronic administration of CB2 agonists in a murine model of bone cancer results in unwanted side effects, and 6) most importantly, offer a novel target for new and innovative therapy for patients suffering from bone cancer due to breast metastases. New treatments are urgently needed that would result in adequate pain relief without the debilitating CNS side effects of current agents, as well as inhibit bone degradation, avoiding painful bone fractures induced by the metastases and result in disease modification. These studies are likely to offer new opportunities for the development of strategies to treat pain resulting from metastasis of breast to bone as well as possible applications to other bone cancers.